Flat conveyor belting is traditionally configured to move through a circuit in a conveyor. The circuit includes a drive pulley, a carryway tension zone, a returnway tension zone, an infeed and an outfeed. In many instances, the outfeed of the conveyor system also serves as the drive pulley. In traditional configurations, the flat conveyor belt is driven through the circuit by means of friction between the bottom surface of the belt and the drive pulley. In order to create sufficient friction to drive the belt along the circuit, the belt must be pre-tensioned.
FIG. 1 illustrates a conveyor 10 running a traditional, pretensioned, flat, friction-driven conveyor belt 60 suitable for transferring products to and from the conveyor. The conveyor includes a drive pulley 30 below the conveyor 10, thereby enabling small diameter rollers in the infeed 21 and the outfeed 22. Small diameter rollers in the infeed 21 and outfeed 22 are desired when, for example, small product is desired to transfer smoothly between conveyors.
As the flat, pretensioned, friction-driven, conveyor belt 60 moves in the direction of arrow 62 from the infeed 21 to the drive pulley 30, it runs along a carryway tension zone 40, as distinguished from a returnway tension zone 50, which extends from the drive pulley 30 to the infeed 21.
The difference between the tension on the conveyor belt 60 at the beginning of the returnway tension zone 50 and the tension on the belt 60 at the end of the carryway tension zone 40 is referred to as the “tension differential,” and their ratio is referred to as the “tension ratio.” The maximum tension differential and the maximum tension ratio depend upon the interplay between the coefficient of friction between the drive pulley 30 and bottom surface of the conveyor belt 60 and the belt arc of wrap in radians around the drive pulley 30. The maximum tension ratio can be calculated as follows:
            T      CT              T      RT        =      ⅇ          (              COF        ×        AW            )      where T represents belt tension, TCT represents the belt tension at the end of the carryway tension zone 40, TRT represents the belt tension at the beginning of the returnway tension zone 50, COF represents the coefficient of friction between the bottom surface of the belt 60 and the periphery of the drive pulley 30, and AW represents the arc wrap of the belt in radians.
In order for the conveyor belt 60 to drive through the circuit of the conveyor 10 of FIG. 1 with no product load, the conveyor belt 60 must be pretensioned. This pretension is referred to as static tension, as opposed to dynamic tension. Pretension is the tension in the conveyor belt 60 that is applied prior to the operation of the conveyor belt. This static tension can be generated in different ways, but most frequently it is generated by extending the effective circuit length beyond the natural length of the conveyor belt 60. The static pretension is present in the conveyor belt prior to operation, when the belt is stationary, and also during operation. Even when product load is added, the predominant tension in the belt 60 is pretension. Therefore, the conveyor belt 60 has significant tension in the returnway tension zone 50.
When product is conveyed along the carryway tension zone 40 of the conveyor 10, the load is increased and, with it, the tension on the belt in the carryway tension zone. As the belt returns through the returnway tension zone 50 of the conveyor, the tension on the belt is reduced.
The maximum tension differential determines the amount of pretension that is required to effectively drive a flat conveyor belt for a given maximum amount of product load. When a flat conveyor belt is being driven without any product load, the actual tension differential will be at the lowest point. The tension on the belt in the returnway tension zone will be similar to the tension on the belt in the carryway tension zone. However, when product load is added, the actual tension differential will increase. The higher the product load, the greater the actual tension differential will become. Therefore, since the maximum tension differential for a flat conveyor belt is limited by the practical limits of arc of wrap and the coefficient of friction, pretension accounts for a significant portion of the tension in the belt at any given time, and in fact is often the majority of the tension in the belt. The result is that when traditional flat conveyor belt is friction driven, the belt experiences high amounts of tension throughout the length of the belt at all times.
An advantage of this inherently needed pretension is that, dependent upon the flexibility of the material construction of the belt, the friction-driven belt can readily conform to various transition geometries at the infeed and outfeed of the conveyor.
However, there are two significant disadvantages to the inherently needed pretension of a friction-driven, flat conveyor belt. The first disadvantage of using the high belt tensions is that misalignment of any component in the conveyor causes large forces to off-track the belt, causing damage to both the conveyor and the belt.
A second disadvantage is that the flat conveyor belt 60 tends to stretch as pretension is applied. In order to limit the amount of stretch in the conveyor belt while maintaining flexibility, fabrics and cords are added to restrict the stretch and enable the belt to operate under high tensions. However these fabrics and cords are a serious harborage point of bacteria and possible pathogens in food processing applications. The common off-tracking of the belt further causes edge fray promoting the exposed fabrics to wick foreign contaminates into the belt where bacteria colonies can grow.
In recent years, a new style of conveyor belting has emerged to counter these and other disadvantages of traditional flat conveyor belts. For example, a positively-driven, low-tension conveyor belt, such as the ThermoDrive® belt available from Intralox, L.L.C., is driven through positive engagement of teeth on the bottom surface of the belt with a sprocket or sprocket-like pulley, instead of pure friction. A positively-driven conveyor belt has a dramatically higher maximum tension differential between the carryway tension zone and the returnway tension zone and therefore the level of pretension is dramatically reduced. U.S. Pat. No. 7,850,562, entitled “Low Friction, Direct Drive Conveyor Belt,” the contents of which are incorporated herein by reference, discloses a method under which such a belt can be driven with no pretension requirements at all. When utilizing the technology described in U.S. Pat. No. 7,850,562, the tension ratio is theoretically infinite. Yet even without this technology, the maximum tension differential is significantly higher for positively-driven, low-tension, toothed conveyor belts than for traditional, friction-driven, flat, conveyor belting and thus the level of pretension required is low for positively-driven conveyor belts.
There are two significant advantages of reducing pretension in a positively-driven conveyor belt. First, the tracking problems associated with misalignment in the conveyor are reduced or even removed. Further, because the pretension is so low, many belts are constructed with no fabric reinforcements at all, which improves food safety and hygiene in food processing applications.
An example of a flexible, endless, positively-driven, low tension conveyor belt suitable for implementing an illustrative embodiment of the invention is shown in FIG. 2. An endless conveyor belt 160 in a typical installation moves around two cylindrical belt-guiding members, illustrated, as sprockets 112 and 114, through a circuit. A first sprocket 114 may be a drive sprocket for driving the conveyor belt, while the second sprocket may be an idle, a driven or slave sprocket 112. The drive sprocket 114 also functions as transition geometry in the outfeed of the conveyor. The belt 160 has an outer surface 111 serving as an article-conveying surface and an inner surface 122 serving as a drive surface. The inner surface 122 includes drive elements, illustrated as teeth 126, preferably spaced equidistantly from each other along the inner driven surface 122. The teeth 126 engage grooves 116 spaced around the circumference of the sprockets 112, 114 to move the belt. The upper span (carryway) 140 of the belt will travel in the direction of arrow 115. The flexible belt 160 wraps around the sprocket 114 and around one or more return rollers, or shoes or drums, in the return path (returnway). The conveyor belt 160 operates at low tension, resulting in substantial catenary sag (not shown) in the returnway tension zone 150. The sprocket in the infeed 112 is of a larger diameter so that the positively-driven, toothed, low-tension, conveyor belt 160 can properly conform to the sprocket in the infeed end of the conveyor.
The belt is made of a resilient material, such as a thermoplastic polymer, an elastomer, or a rubber, and is flexible along its length.
To transfer products between two endless conveyor belts, the belts must be placed close together to minimize the gap between the conveyor belts at the transfer point. Small nosebars, shoes or other structure are usually used at the transfer locations to allow the ends of the belts to be placed in close proximity to each other.
A disadvantage of positively-driven, toothed, low tension, conveyor belting is that without sufficient tension in the belt it does not readily conform around transition points at the infeed that are smaller than the arc of natural curvature in the belt as it transitions around the infeed. Small transfers in ThermoDrive® and other low-tension, positive drive endless conveyor belts are often difficult, because the lack of tension prevents the belt from conforming to a small nosebar or other infeed structure.
FIG. 3A is a simplified schematic cross-sectional representation of a conveyor 200 running a positively-driven, toothed, low-tension, conveyor belt 260, through a conveyor circuit in which small diameter cylindrical members are used at the infeed and outfeed. The conveyor circuit includes a carryway 240 and a returnway 250. Smaller diameter sprockets 211, 231 are used in the infeed 210 and outfeed 230 to facilitate transfer of products onto and off the conveyor belt 260. However, problems arise when the sprocket 211 at the infeed 210 is smaller in diameter than the arc of the natural curvature of the belt 260 as it transitions around the infeed 210, as shown in FIGS. 3A and 3B. The result of this smaller diameter sprocket in the infeed 210 is that the conveyor belt 260 protrudes beyond the plane of the belt circuit creating a ridge 262 along the width of the conveyor belt 260 at the infeed 210, which makes transfer of product onto the belt difficult.
To resolve these drawbacks, users of positively-driven, toothed, conveyor belting have resorted to adding more pretension than is required to drive the belt, in order to achieve the desired conformation around the infeed roller, thus minimizing the tracking benefits and non-fabric-reinforced sanitary benefits that could otherwise be achieved in a non-pretensioned belt.
The amount of pretension required to maintain belt conformity to specific transition geometries at the infeed is greater than the amount of pretension required to achieve belt conformity at the infeed when the belt is installed. This is because when product load is added and the tension differential increases, the added tension on the belt in the carryway tension zone results in some amount of conveyor belt elongation. This additional belt length is generally found in the returnway tension zone, resulting in a tension in the belt as it encounters the infeed that is lower than the pretension initially applied. To maintain belt conformity around small transition geometries at the infeed by means of pretension, the toothed, positively driven conveyor belt will often be pretensioned beyond the pretension level required to drive the conveyor belt.